I know this was already asked here but I didn't read an answer that really helped answer my thoughts on this. The biggest argument there is that Vader saw Luke with Obi-Wan and put 2 and 2 together. However.... During this time, it was fairly obvious Luke was nothing more than a padawan. Vader knows that he killed Obi-Wan but continues on in Return of the Jedi to say "I see Obi-Wan has taught you well..."
To me, this signifies that Darth Vader knew Obi-Wan was still able to communicate to Luke even after death. If that was the case... wouldn't he also be able to understand Yoda or other Jedi Council members would be able to do the same? Vader would naturally assume other Jedi to be killed due to Order 66, but he should also know that if Obi-wan can manifest through the force that so should Yoda. I am more surprised that the scene didn't go.. "Your training as a Jedi has been complete. You couldn't have done this alone... who taught you?" Bad dialog sure, but the principle remains.
So why does Vader say with fact that Obi-Wan taught him, and not with question or wonder about who finished his training/provided the training knowing that a few years earlier he killed Obi-Wan when Luke had minimal Force knowledge?